[DOP-14058] Improve Kafka documentation

[dolfinus]

Change Summary

• Move parse_column and serialize_column examples to docstrings. Example
• Move example of Kafka DataFrame to separate page, and merge it with serialization/deserialization examples. See

Related issue number

Checklist

• [X] Commit message and PR title is comprehensive
• [X] Keep the change as small as possible
• [X] Unit and integration tests for the changes exist
• [X] Tests pass on CI and coverage does not decrease
• [X] Documentation reflects the changes where applicable
• [ ] docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change (see CONTRIBUTING.rst for details.)
• [x] My PR is ready to review.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.71%. Comparing base (aa2b753) to head (dccd8c5). Report is 71 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #276      +/-   ##
===========================================
+ Coverage    94.69%   94.71%   +0.02%     
===========================================
  Files          204      204              
  Lines         8068     8068              
  Branches      1410     1410              
===========================================
+ Hits          7640     7642       +2     
+ Misses         305      304       -1     
+ Partials       123      122       -1     

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